You’ll be able to use your gamertag on an Xbox One and an Xbox 360 at the same time

Stevivor -- "As part of IGN’s latest “Ask Microsoft Anything About Xbox One,” Chief Xbox One Platform Architect Marc Whitten confirmed that gamers will be able to simultaneously log into an Xbox 360 and an Xbox One.

"'You’ll be able to be signed on with the same gamertag on both an Xbox 360 and an Xbox One console at the same time,” Whitten confirmed with IGN. “However, like today, you can only be signed into one Xbox 360 and one Xbox One at a time.'"

It's not about whether or not it's "mind blowing," but rather that it is a nice feature to have regardless. People, gamers especially, always try to downplay and shift the interest/importance of topics by using the "this did it first argument," but it really is irrelevant.

Take a game like Portal. I think a lot would agree that the game was fresh and innovate, right? well, it wasn't unique, because it was inspired and essentially the spiritual successor of a previous game - a free PC game from 2005 called "Narbacular Drop." Does this suddenly void everything Portal accomplished and introduced to gamers? No. The same goes for the ability to sign in on 360 and X1, despite if Sony "did it first."

Not exactly sure but I do know achievement whores can use if to their advantage. One person on One getting achievements and one on 360 getting achievements for the same profile, other than that.....idk

As has been stated, players can plug a 360 into the hdmi-in port of the X1 and play 360 games on the X1. In this regard, a player could could be signed into XBL on both X1 and 360, and switch between games for both using only one system.